The invention relates to the true-to-quantity introduction of ions of a wide mass range into a very strong magnetic field in the direction of the magnetic field lines to an ion storage device, for example: a measuring cell in an ion cyclotron resonance mass spectrometer. The development of magnetic field generators with superconducting solenoids for very strong magnetic fields is advancing very rapidly. This type of magnet is used both for nuclear magnetic resonance spectrometry (NMR) and also for ion cyclotron resonance mass spectrometry (ICR-MS). For the latter, magnets with field strengths of 7, 9, 12 and 15 Tesla are now available and supplied. Instruments with 21 Tesla magnets are planned. Several of the performance specifications for ICR mass spectrometers increase linearly with the field strength. Some other important performance specifications, such as the resolution or the ion collection capacity of the measuring cells without interfering with the scan, even increase with the square of the field strength, making it easy to understand why researchers are trying to achieve higher field strengths.
Every magnet for an ICR mass spectrometer has a so-called open bore (also called “room-temperature bore”), usually with a diameter of around eleven centimeters, which allows access to the inner region with the highest and most homogeneous field strength. The axis of the bore coincides with the axis of the magnetic field. A long, tubular vacuum recipient, which contains the measuring cell for analyzing the ions in the form of an ion storage device, is inserted into this bore. The aim of the investigations is usually to determine the mass of the ions, which is obtained by measuring the circular cyclotron motions which an ion assumes after appropriate excitation. A very good vacuum of better than 10−6 Pascal is required to keep the ions moving freely and without collisions over periods of several seconds.
In the past, magnets of this type were passively shielded by several layers of thick iron sheets, which meant that 12 Tesla magnets weighed more than 15 tons. Nowadays such magnets use active shielding. This means that an inner coil system and an outer coil system are used to feed most of the field lines of the magnetic field of the inner solenoid back through the outer coil system, thus producing only very small magnetic fringe fields at the entrances and exits of the bores. This gives a very steep magnetic field increase at the entrance of the magnet. The superconducting coils are located in helium cryostats, which, in turn, are usually enclosed in liquid nitrogen cryostats. The walls of the bores are at room temperature; the magnets are therefore technically very complex to manufacture.
The steep increase of the magnetic field leads to difficulties when introducing the ions, which are generated outside the magnetic field and introduced into it. Only ions which are injected exactly on the axis of the magnetic field and its fringe field have a chance of reaching the measuring cell; all other ions injected either at a slight angle or slightly off-axis are reflected by the fringe field as if they were in a magnetic bottle. Asymmetric distortions of the fringe field mean that no ions at all can be injected. Unless special measures are taken, it sometimes requires several days of adjustments until an alignment of the recipient to the magnet is achieved which allows a satisfactory number of ions to reach the measuring cell. This adjustment has to be repeated after each new insertion of the recipient unless special measures are taken to maintain the alignment.
About two decades ago, a way of greatly simplifying this adjustment for magnets of medium field strength was described. This used an RF quadrupole rod system (R. T. McIver, U.S. Pat. No. 4,535,235). The system, consisting of four long pole rods, extends through the magnetic field increase to the measuring cell in the homogeneous magnetic field. The two phases of an RF voltage are applied alternately to the pole rods of the quadrupole system, in whose interior a radially focusing pseudopotential is produced. In this way, the ions can be more easily and reproducibly guided from the outside through the fringe field to the measuring cell.
If ions of a very wide mass range are to be transported more or less uniformly into the measuring cell, then it is favorable, according to recent research, to increase the number of poles when using magnets with higher magnetic field strengths, i.e., to change from quadrupole rod systems to hexapole, octopole, or even higher multipole systems. But even then, this ion guide does not work for very strong, short magnets, especially for light ions. Heavy ions are transported fairly satisfactorily, but many light ions do not arrive at the strong magnetic field.
Research has shown that the light ions are lost in the region of the magnetic field increase where their cyclotron frequency just equals the RF frequency of the pole rod system. The cyclotron motions of the ions are resonantly excited by the electric fields, which have components at right angles to the magnetic field lines inside the pole rod systems. As a result, the ions are moved out of the system until they collide with the pole rods. The RF fields can also excite harmonics of the ion motion, or the cyclotron motion of harmonics of the RF. In any case, no true-to-quantity transport of the ions of different masses takes place.
To achieve maximum sensitivity, the ions under investigation are usually collected in a temporary store outside the magnetic field and introduced into the magnetic field from this temporary store. The easiest method is to accelerate the ions simultaneously as an ion cluster from the temporary store, and to transfer them to the measuring cell. The capture of the ions in the ion storage device acting as a measuring cell in the magnetic field is greatly simplified if the ions of all masses enter with low energies and at the same time. The aim is to achieve entrance energies of approx. 0.3 electron-volts. The long path from the ion supply to the measuring cell, however, causes a temporal mass dispersion of the ion cluster which has been transferred, so that the ions arrive at the measuring cell separated according to their mass: first the lighter and faster ions, then increasingly the heavier ones. This temporal mass dispersion can be greatly reduced, but not eliminated, by strongly accelerating the ions from the temporary store and decelerating them before they enter the measuring cell. The large overall length of strong magnets, which represents a long flight path, therefore presents a further problem for a highly efficient, and also true-to-quantity, capture of the ions from the temporary store.
The greatest successes with ICR mass spectrometry have been achieved in the field of proteomics, and especially in the field of “top-down analysis” of proteomes, where the masses of hundreds or even thousands of digest peptides are simultaneously analyzed in the measuring cell and subsequently assigned to the undigested proteins of the proteomes. For reasons which are not yet fully understood, the larger the number of different types of ions in the measuring cell, the better the ICR mass spectrometry operates. Accuracies of much better than one millionth of the mass can be achieved in the mass determination; no other type of mass spectrometry can measure this accurately. This application (and also other methods in proteomics) works optimally when both the ions of individual, cleaved amino acids (so-called immonium ions) with masses from 50 Daltons upwards and peptides with mass-to-charge ratios of approximately 5,000 Daltons can be measured together. It should therefore be possible to introduce ions of the mass range of 1:100 into the measuring cell. The velocities of these ions extend over a range of 1:10 at the same kinetic energy. This data explains the problem for the true-to-quantity, efficient introduction of the ions into the measuring cell.
The term “mass” here always refers to the “mass-to-charge ratio” m/z, which is the only parameter of importance in mass spectrometry, and not simply to the “physical mass” m. The number z is the number of elementary charges, i.e., the number of excess electrons or protons which the ion possesses, which act externally as the ion charge. All mass spectrometers without exception can measure only the mass-to-charge ratio m/z, not the physical mass m itself. The mass-to-charge ratio is the mass fraction per elementary ion charge. The terms “light” and “heavy” ions here analogously refer to ions with a low and high mass-to-charge ratio m/z respectively. Similarly, the term “mass spectrum” always relates to the mass-to-charge ratios m/z.